1. Field of the Invention
The invention relates to a method and apparatus for straightening a coupling shaft, particularly the coupling shaft of a three joint assembly vehicle drive shaft including a center shaft support bearing.
2. Summary of Related Art
Vehicle drive shaft assemblies are comprised of one or more tubular members having elements such as a yoke of a universal joint, a bearing stub or one component of a slip joint fitted at each end thereof. Such elements are usually pressed into the hollow interior of the tubular member(s) and secured by a circumferential weld joint effected between the end of the tubular member and a surface of the inserted element. The drive shaft assembly is utilized as a torque transmitting component of the power train and is most commonly located under the vehicle between the transmission or other gear box and a drive axle.
In some applications, where the length and speed of operation required of the drive shaft would cause excessive radial misalignment (commonly termed "run-out") of a single tubular member, three joint drive shaft assemblies have been utilized. Such drive shaft assemblies include two interconnected tubular members--a coupling shaft connected to the transmission, and a drive shaft connected to the drive axle. The coupling shaft has a yoke for a universal joint mounted at the end connected to the transmission, and a midship tube shaft mounted in the other end. The midship tube shaft is supported by a center shaft support bearing assembly attached to the vehicle frame and is connected, often by means of a slip yoke assembly, to the drive shaft.
In operation, it is important that the tubular members of the drive shaft assembly remain relatively straight and in balance. A lack of straightness or radial alignment may cause noise, vibration and harshness problems in the vehicle. If excessive, radial misalignment may result in failure of the tubular members of the drive shaft assembly.
To avoid such problems, the typical drive shaft fabrication process includes the straightening and subsequent balancing of the tubular members. The straightening step is accomplished by placing the shaft in a straightening device having a hydraulic press which presses against the shaft to achieve radial alignment. The balancing operation requires a balancing machine having an intricate set-up of spindles, shafts and bearings to provide smooth high-speed rotation of the shaft so that vibration sensors may detect out of balance areas on the shaft.
The coupling shafts of three joint drive shaft assemblies have heretofore been straightened prior to mounting on the center shaft support bearing assembly as follows. The tube yoke with journal cross or flange and the midship tube shaft are affixed in the opposing ends of the tubular member of the coupling shaft prior to the straightening operation. The coupling shaft is rotatably mounted in a straightening press off of the open joint or flange in one end, and off of the end of the center of the midship tube shaft at the other end. The coupling shaft is rotated and run-out readings are taken at the bearing surface of the midship tube shaft, near the end of the tubular member. The coupling shaft is then positioned so that the hydraulic press applies a force against the shaft to permanently deflect the same to achieve radial alignment. This process is typically repeated at various points along the tubular member of the coupling shaft. After the straightening operation, the coupling shaft is balanced, either before or after being assembled to the drive shaft. The more effective the straightening operation, the easier and faster the coupling shaft or entire drive shaft assembly is to balance.
The above method is adequate for straightening a drive shaft which is mounted at each end by a universal joint yoke or flange. It is disadvantageous for straightening coupling shafts, however, in that the coupling shaft is straightened with reference to a hypothetical center line created by supporting the coupling shaft at its two ends. The actual center line established after the coupling shaft has been mounted on the vehicle may in fact be quite different from the hypothetical center line used during such a straightening operation. This is because the coupling shaft is actually mounted to the vehicle off of the universal joint yoke at one end, and off of the bearing surface of the midship tube shaft which is supported in the center shaft support bearing. Additionally, run-out readings have heretofore been taken at the bearing surface of the midship tube shaft, which is adjacent the end of the tubular member. However, the run-out may be much greater at the end of the midship tube shaft than that which is measured at the bearing surface. Since the coupling shaft mates with the drive shaft at the end of the midship tube shaft, it is the run-out experienced at that point which has the greatest effect on the alignment of the overall drive shaft assembly.